The invention discloses an enteromorpha polysaccharide having immunocompetence and a preparation method thereof. The enteromorpha polysaccharide having immunocompetence is a polysaccharide HT-II. The enteromorpha polysaccharide having immunocompetence is a white floccule, can be well dissolved in water and dimethyl sulfoxide, has relative molecular weight of 96kDa, and mainly comprises glucose and mannose, wherein a ratio of rhamnoside to xylose to mannoside to glucose is 36: 20: 6: 38. The preparation method comprises the following steps of breaking down, carrying out extraction at a temperature of 90 DEG C by water, filtrating, merging the extract, concentrating, carrying out precipitation by ethanol, removing proteins, carrying out separation and purification, and carrying out freeze drying to obtain the purified enteromorpha polysaccharide having immunocompetence. The preparation method is simple, has stable performances, a low cost and a large medical value, can reduce cholesterol content and can improve immunity. The preparation method adopts extraction, protein removal and purification processes, and scientific experimental means, is a novel method for controlling enteromorpha, and provides a novel approach for controlling enteromorpha. The preparation method of the enteromorpha polysaccharide having immunocompetence develops enteromorpha medical values and purposes, controls environmental pollution, reduces a clean-up cost, improves economic benefits and is an ideal special preparation technology for enteromorpha control and integrated utilization.